Downing Street today strenuously denied claims that Gordon Brown had objected to Boris Johnson, the mayor of London, sitting next to him at the memorial service for Damilola Taylor held in Southwark cathedral last November.

Johnson told the Commons home affairs select committee today that he had been asked to switch seats at the request of the prime minister. The issue came up because Johnson was trying to explain how he came to be talking to David Cameron, the Tory party leader, on the day that shadow immigration spokesman Damian Green was arrested.

Johnson told the cross-party panel of MPs investigating the police's actions relating to Green's arrest that Brown had been "appalled" at the idea of sitting next to him so the mayor had ended up moving "some distance into the back row".

"So any conversation that might have taken place between me and the leader of the opposition was made very perfunctory thanks to the sensitivities of our great leader," Johnson told the MPs.

Downing Street this afternoon flatly denied the claim.

The prime minister's spokesman said: "That is categorically not true. There was never any instruction or suggestion that he should or should not sit next to the mayor."

But Johnson's spokesman later insisted that the mayor was told by organisers on the eve of the memorial service that plans had changed and that they now wanted him in the second row because "they had come under a bit of pressure" from the PM's office. "The mayor stands by his recollection of events," said Johnson's spokesman.